Investigators say that a retired accountant's shooting rampage was meticulously planned and included specially modified weapons and surveillance cameras aimed to spy on any police intruders who would try and stop his murderous deluge.

At a late-night press conference Tuesday, authorities said Stephen Paddock made his attack even more deadly by adding more lethal components to his weapons. He had devices attached to 12 semiautomatic rifles that allowed them to mimic fully automatic gunfire.

Bureau of Alcohol, Tobacco and Firearms Special Agent in Charge Jill Schneider said Paddock had nearly 50 guns — a combination of rifles, shotguns and pistols — in three locations.

The gun attachment that mimics automatic gunfire is a little-known device called a “bump stock” that is not widely sold. The stocks have been around for less than a decade, and Schneider said officials determined they were legal.

Paddock, who killed 58 people and eventually himself, also injured 527 others in what is the deadliest mass shooting in modern U.S. history.

He also used surveillance cameras to monitor police approaches to his room at the Mandalay Bay high-rise — including a camera he positioned in the peephole of the door.

“I anticipate he was looking for anybody coming to take him into custody,” Sheriff Joseph Lombardo said.

During the Sunday night rampage, a hotel security guard who approached the room was shot through the door and wounded in the leg.

“The fact that he had the type of weaponry and amount of weaponry in that room, it was preplanned extensively,” the sheriff said, “and I’m pretty sure he evaluated everything that he did and his actions, which is troublesome.”

While Paddock's motive remains unclear, authorities were putting together a more complete picture of his work history and his past. The killer worked as a letter carrier for the U.S. Postal Service, an IRS agent and in an auditing department over a 10-year period.

A spokeswoman for the Office of Personnel Management told The Associated Press on Tuesday that Stephen Paddock’s employment included about two years as a mail carrier from 1976 to 1978.

After that, he worked as an agent for the Internal Revenue Service for six years until 1984. And then he worked a defense auditing job for about 18 months.

He graduated from college in 1977 from Cal State Northridge and also worked for a defense contractor in the late 1980s.

President Trump, as he left Washington for a trip to storm-ravaged Puerto Rico, briefly referenced Paddock, whose carnage Sunday night left 59 people dead and more than 500 injured.

"He was a sick man, a demented man with a lot of problems, I guess, and we are looking into him very, very seriously," Trump said. "We are dealing with a very, very sick individual."

The president also said the administration would be "talking about gun laws as time goes by," and he lauded the efforts of Las Vegas law enforcement.

"How quickly the police department was able to get in (to Paddock's hotel room) was very much a miracle," he said. "They've done an amazing job."

Country music star Jason Aldean was performing when Paddock opened fire from the window of his 32nd-floor hotel room overlooking the Route 91 Harvest music festival. Aldean, who was not injured, tweeted a statement Tuesday saying he has been overwhelmed with emotion since the attack.

"Something has changed in this country and in this world lately that is scary to see," Aldean wrote. But he also added that it is "time to come together and stop the hate."

Stories of heroism continued to emerge. One minute Marine veteran Taylor Winstonwas dancing the two-step at the concert, the next he was commandeering a pickup and shuttling the wounded to hospitals before ambulances arrived. Dawn-Marie Gray, a concertgoer who happened to have seven years experience as a paramedic, found herself providing CPR and tying tourniquets.

When Nick Jones, 30, of Vancouver, Wash., heard about the shooting, he drove 20 hours to help. He offered to donate blood, but the need had been met. He offered to volunteer at hospitals, but they had plenty of help. Even the family assistance center was overwhelmed with volunteers.

“I was worried that no one would do anything,” Jones said Tuesday. “I was happy to be proven wrong.”

Assistant Sheriff Todd Fasulo pleaded for patience during the investigation, saying authorities were "hunting down" every clue to learn more about Paddock.

Several minutes after the carnage began, officers who blasted into the room found Paddock's body. His brother Eric, who lives in Florida, says his multimillionaire brother was a big spender at casinos and often received free meals and rooms there. Eric Paddock also said he believed his brother owned a couple guns.

Police found 23 guns, including semiautomatic rifles, in Paddock's room at the Mandalay Bay Resort and Casino. They found 19 more at his home in Mesquite, Nev., 80 miles northeast of here. And in yet another home owned by Paddock in Reno, authorities found five handguns, two shotguns, and a plethora of ammunition.

A federal law enforcement official, who was briefed on the matter but not authorized to comment publicly, said police also found two tripods positioned at the hotel windows in what appeared to be a fully equipped sniper’s nest.

While the investigation intensified, authorities touted the city’s resilience in the aftermath of the tragedy. Fully stocked blood banks were soon turning away would-be donors. A GoFundMe page collected $3.3 million from almost 50,000 donors in little more than 24 hours. Overall, donations for the victims have exceeded $3.7 million, Commissioner Stephen F. Sisolak said Tuesday afternoon.

Pieces of normalcy slowly returned to the iconic Strip. Club promoters hawked 2-for-1 deals, tourists took selfies and police cracked down would-be criminals.

“Do you know how disrespectful it is to come out here and deal drugs the day after 50 people died one block away?’’ a bike cop snapped at a man in handcuffs Monday night. “Can’t you take one day off?’’

Derek Koerner, who identified himself as a licensed club promoter, said he was encouraged by the sizable crowds — and particularly grateful for the SUV full of men interested in his 2-for-1 deals on strip clubs. A $40 cover for a two-drink minimum gets the buyer into the second strip club for free.

“There’s more people out here than I thought there’d be,’’ said Koerner, 48. “People seem to be upbeat and have a good energy. People seem to be moving on with their lives.’’

But less than a mile away, the mood was noticeably different. Dozens of people, some arriving with candles and flowers, returned to the scene of the horrific crime, marked by yellow police tape.

Herman Gold and his ex-wife, Sara, stood near the concert grounds and explained how they escaped the hail of bullets and why they came back.

“I just want closure,’’ Herman Gold said. “Why? What’s the reason that on that particular night, one individual would sacrifice everyone else for his insanity?’’

Joe Larsen, standing at the same corner and wearing a VIP wristband from the Aldean concert, expressed gratitude.

Larsen, 49, who flew in from New Jersey for the concert, said that he watched the Twin Towers fall in 2001, lost his wife to cancer in 2015 and, on Sunday night, feared he was about to leave his son and daughter without a parent.

He said he saw a young woman die from a bullet to the head before he called his father during the bedlam.

“I told him, ‘I don’t know if going to make it. Tell my kids to live a good life’ ” Larsen recalled.

As he and others shared their stories and wiped away tears, police officers closely monitored pedestrians, especially the wayward ones.

“Sir, get out of the street,’’ one officer barked. “It’s the last time I’m telling you.’’